Measurement Uncertainty | Metrology Matters
Summary
TLDRThis script delves into the intricacies of measuring an aluminum part's diameter, highlighting the challenges of achieving exactness due to inherent measurement uncertainty. It explains the concepts of repeatability and reproducibility through GR&R studies, emphasizing the difference between precision and accuracy. The importance of adhering to standards, such as ASME B89.7, is underscored to manage uncertainty effectively. The script concludes with a practical example of how environmental factors and handling can significantly impact measurement outcomes, advocating for a deeper understanding of these uncertainties.
Takeaways
- 📏 The script discusses the challenge of measuring the exact diameter of an aluminum part due to inherent measurement uncertainty.
- 🔍 Two different tools provided measurements with different levels of precision, showcasing the variability in measurement results.
- 🤔 It's impossible to ascertain the true exact value of a measurement due to the limitations of measurement methods and tools.
- 🔬 The concept of measurement uncertainty is introduced as the gap between the true value and the measured value.
- 📈 The GR&R (Gauge Repeatability and Reproducibility) study is mentioned as a method to estimate measurement repeatability but not accuracy.
- 🎯 The goal of measurement is to achieve both high accuracy (closeness to the true value) and precision (consistency of repeated measurements).
- 🛠️ Sophisticated technology can help in reducing measurement uncertainty by allowing for more precise measurements.
- 📚 The importance of following strict standards in metrology is emphasized for managing uncertainty effectively.
- 📋 Standards like ASME B89.7 provide guidance on managing uncertainty, including measurement planning and estimating risks.
- 🔄 Factors contributing to measurement uncertainty include the environment, equipment, setup, and the object being measured, among others.
- 🧠 Understanding the sources of uncertainty is crucial for having confidence in the accuracy of reported measurements.
Q & A
What is the main challenge in measuring the diameter of an aluminum part accurately?
-The main challenge is that no measurement method can find the exact true value due to the inherent measurement uncertainty. Even with precise tools, there's always room for even closer approximation.
What is meant by 'measurement uncertainty'?
-Measurement uncertainty refers to the dispersion of measured values around the actual measure. It's the gap between the true value of a dimension and the measured value that can be obtained.
What does GR&R stand for and what is its purpose in measurement?
-GR&R stands for Gauge Repeatability and Reproducibility. It is a study where different operators measure the same parts repeatedly to estimate the repeatability of the measurement process, but it doesn't necessarily indicate accuracy.
Why is it important to have both accuracy and precision in measurements?
-Accuracy ensures that measurements are close to the true value, while precision ensures that repeated measurements are consistent and close together. Both are necessary to reduce measurement uncertainty.
How does technology contribute to reducing measurement uncertainty?
-Sophisticated technology allows for more precise measurements, generally meaning that instruments capable of measuring smaller units are likely to have less deviation from the correct value.
What role do standards play in managing measurement uncertainty?
-Standards, such as those created by the American Society of Mechanical Engineers, provide guidance for managing uncertainty by addressing measurement planning, estimating uncertainty, and traceability, among other considerations.
What are some factors that can cause measurement uncertainty?
-Factors include the environment, reference elements, measurement equipment, setup, software, calculations, the object being measured, how the target characteristic is defined, and physical constants, among others.
Why is it useful to be certain about measurement uncertainty?
-Being certain about measurement uncertainty gives more confidence in the accuracy of reported measurements, as it provides a concrete understanding of how and why measurements may be inaccurate.
How does the material's property, such as aluminum's expansion rate, affect measurement uncertainty?
-Material properties like expansion can significantly affect measurement uncertainty. For example, aluminum's expansion rate can change its size with temperature, potentially increasing the uncertainty if not accounted for.
What additional measurement considerations are mentioned in the script that could affect uncertainty?
-The script mentions the need to consider the object's roundness and the importance of reviewing the measurement process, which can also contribute to uncertainty if not properly addressed.
How does the script illustrate the concept of repeatability versus accuracy?
-The script illustrates this by showing that even if a measurement method is highly repeatable (consistently hitting the same value), it can still be inaccurate if those values are not close to the true value.
Outlines
📏 Measurement Uncertainty and Precision
The script discusses the concept of measurement uncertainty, emphasizing that no measurement method can determine the exact true value of a physical object's dimension. It explains the difference between repeatability and accuracy, highlighting that high precision does not guarantee accuracy. The importance of gauge repeatability and reproducibility (GR&R) studies is mentioned as a way to estimate measurement uncertainty. The script also touches on the role of standards in managing uncertainty, referencing the ASME B89.7 standards for guidance.
Mindmap
Keywords
💡Diameter
💡Measurement Uncertainty
💡True Value
💡Repeatability
💡Reproducibility
💡Precision
💡Accuracy
💡Metrology
💡American Society of Mechanical Engineers (ASME)
💡Expansion
💡Roundness
Highlights
Measurement of an aluminum part's diameter with a specified uncertainty of ±408 micro inches.
Verification of the measurement using a different tool yielding a different result.
The impossibility of achieving an exact true value in measurements due to inherent uncertainties.
The concept of measurement uncertainty as the gap between the true value and the measured value.
Gauge repeatability and reproducibility (GR&R) study as a method to estimate measurement uncertainty.
The distinction between repeatability and accuracy in measurement results.
The potential for high precision measurements to be inaccurate if they consistently miss the true value.
The goal of increasing both accuracy and precision to reduce measurement uncertainty.
The role of sophisticated technology in achieving more precise and accurate measurements.
The importance of following strict standards in metrology to manage measurement uncertainty.
The American Society of Mechanical Engineers' B89.7 standards addressing uncertainty considerations.
Factors contributing to measurement uncertainty, including environmental conditions and equipment.
The significance of understanding the sources of measurement uncertainty for more reliable results.
The impact of physical properties, such as thermal expansion, on measurement uncertainty.
The need for comprehensive review of measurement techniques to ensure accuracy and precision.
Transcripts
I have an aluminum part and I want to
measure its diameter
0.2315 inches plus or minus 408 micro
inches let's check that with another
tool
this one says it's 0.2293 plus or minus
6.875 micro inches so what's the real
value
impossible to be certain
for every measure and the quantity we
intend to measure there's technically a
true exact value if we're talking
dimensional measurement any physical
object has an actual size we're just
trying to figure out what it is the
problem is that no measurement method
could ever find the exact True Value get
pretty close but that's about it no
matter how precise our tools may be
there will always be a little more room
to be even closer to the right answer
our goal is just to confidently get as
close as possible the gap between the
true value of a dimension and the
measured value we can obtain is the
measurement uncertainty the most
accepted way to describe measurement
uncertainty is through a gauge
repeatability and reproducibility study
typically shortened to grnr a number of
different operators take a number of
different parts that each repeatedly
measure those parts and then compare the
results this does a great job of
estimating our repeatability but it
doesn't necessarily speak at all to our
accuracy C measurement uncertainty
refers to the dispersion of measured
values around the actual measure Ram So
in theory if your aim is off by roughly
the same amount each time but in
slightly different directions the
average from your repeated trials should
lead you more or less at the actual
Target in the middle of that dispersion
but it's entirely possible to have
really great repeatability hitting very
close to the same measurement every time
and still be inaccurate High Precision
can just mean being repeatedly wrong
instead we want to get really close to
the true value of the measureand
directly that just kind of land
somewhere all around it extrapolate and
hope for the best and on top of that we
want reliable repeatability measurements
that consistently Land close together in
repeated trials we want to increase both
our accuracy and our Precision in other
words we want to shrink the measurement
uncertainty one of the ways we do this
is through increasingly sophisticated
technology that's up to the task of
taking such careful measurements
generally speaking the smaller the units
an instrument can reliably measure the
smaller the amount it's likely to to
stray from the correct value right I
mean you wouldn't say that you can
measure down to the micrometer if your
measurement tool could be off by a whole
dang millimeter good Precision helps
narrow the accuracy window another
crucial way that we handle measurement
uncertainty is you guessed it by
following strict standards here on
Metrology matters we can't overstate the
importance of Standards the guidance
from standards for managing uncertainty
really help make things less uncertain
for example the American Society of
mechanical engineers created the b89.7
theories of Standards six short
documents addressing several important
uncertainty considerations these include
measurement planning estimating
measurement uncertainty building
uncertainty statements estimating
measurement risk decision rules for
accepting and rejecting components and
traceability these standards also
address why any of this uncertainty
happens in the first place what causes
us to stray from the measure end turns
out basically everything some factors
include the environment you're working
in your reference element your
measurement equipment the measurement
setup software and calculations doing it
the object you're measuring how the
target characteristic is defined for
measuring procedure physical constants
and a whole lot more it's a lot to keep
track of but very important to
understand overall having reliable and
concrete knowledge of how inaccurate our
measurements are and why gives us more
confidence in the accuracy of what we
report in other words it's very useful
to be certain about our uncertainty
with all that in mind let's take one
more look at that aluminum part
now it says it's like 15 micro inches
bigger than it was before all right I've
had it in my pocket this whole time and
aluminum expands at 25 parts per million
per degree Fahrenheit so I probably like
quadrupled the uncertainty I haven't
even tested its roundness I guess I also
need to review more about the measure
end
5.0 / 5 (0 votes)